CMOS image sensors may use a rolling shutter, in which pixels in a pixel array are electronically shuttered line by line. The term “electronic shuttering” relates to the electronic control of a pixel to define an exposure period in which the pixel is exposed to light for the capture of an image. The end of the exposure period may be defined by the readout of the pixel. Alternatively, the exposure period may be terminated prior to readout. The total time from the start of the exposure period to the end of readout is known as the integration time.
Sensors which use a rolling shutter distort images of moving objects because the object moves between the times at which successive lines are electronically shuttered. The distortion may take the form of time-displacement artefacts, as shown in FIG. 1 and FIG. 2.
A mechanical shutter can be used to overcome the problem, but adds to the cost, complexity and size of the image-capturing apparatus.
Another way to avoid the problem is to use a global shutter, in which all pixels are electronically shuttered simultaneously. This is usually followed by a readout stage in which pixels are read out line by line. Simultaneous readout is not economical since it requires a separate readout structure and line from each pixel to output or memory.
The global shutter method suffers due to increased dark-current error. Dark current is the current generated in a pixel when no light is incident on the pixel. When the exposure period begins, charge starts to generate at a rate depending on the amount of light striking the pixel. This charge is generated only during the exposure period. However, charge is also accumulated due to dark current, leading to an inaccurate reading. The accumulation of charge due to dark current occurs throughout the integration time, i.e. it continues during and after the exposure period and does not end until the integration value of the pixel is read out. As the pixels are read out line by line, the integration time for the last pixel is longer than for the first pixel, and so the dark-current error gradually increases from one end of the image to the other. This is unlike a rolling shutter in which every pixel has an identical integration time. Furthermore, the integration time when using a global shutter can be higher than when using a rolling shutter, leading to a more significant dark-current error.
The listing or discussion of a prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.